1. Field of the Invention
The present invention relates to a composite gas separation membrane and its preparation method. The gas separation membrane of the present invention which is produced by blending thermoplastic resin, liquid crystalline polymer and compatibilizer to control the dispersion state of the liquid crystalline polymer in the thermoplastic resin and adhesion at the interface, and then biaxially drawing the obtained blended mixture is capable of increasing a selective permeation rate in such a manner that the dispersed liquid crystalline polymer is morphologically altered to serve as an obstacle to a gas permeation, by which the permeation distance of the gas is elongated, which in turn enhances the selective permeability over the so-called trade-off limit. More particularly, a liquid crystalline polymer which is hardly permeable to a gas is dispersed in a thermoplastic resin, and at this time, an appropriate amount of compatibilizer is added so as for the liquid crystalline polymer to have a uniform size and well dispersed state, and then, the mixture is biaxially drawn in a film blowing process to obtain a thin film, whereby, the liquid crystalline polymer phase evenly dispersed and morphologically controlled in the film works as an obstacle to diffusion of gases, to thereby morphologically altering path length of gases. The gases are separated according to interaction difference which induces diffusion time difference during which the gases pass the deformed path by using diffusion constant difference depending on the size of the gas molecule, thereby exhibiting a selectivity of a gas higher than the limit value.
2. Description of the Background Art
Polymeric separation membranes are used for various gas separation processes which are technologically important such as separation of air, separation of carbon dioxide from a natural gas, or separation of nitrogen or hydrocarbon and hydrogen in a petrochemical process.
A basic factor determining the separation performance of separation membranes for a pair of gases (i.e., oxygen/nitrogen, carbon dioxide/methane or hydrogen/nitrogen, etc.) is a permeability constant and a selectivity. Permeability constant is typically obtained by dividing the multiplied value of a gas concentration difference and the thickness of separation membrane by a pressure difference of both sides of the separation membrane. Selectivity is a ratio of permeability of two gases. That is, when the selectivity ratio is represented by A/B, xe2x80x98Axe2x80x99 is the permeability of high permeable gas and xe2x80x98Bxe2x80x99 is the permeability of low permeable gas.
High performance separation membranes are desired to have a high permeability and a high selectivity, because a high permeability allows to reduce an area of separation membranes required for separating a certain amount of gas and a high selectivity allows to enhance the purity of a produced gas.
However, generally, if the separation membranes have a high permeability, it has a low selectivity, while if a separation membrane has a high selectivity, it has a low permeability.
According to Robeson""s observation, as to separation of several pairs of gases, most polymer separation membranes have a negative slope in the correlation between the selectivity and the permeability (Journal of Membrane Science, Vol. 62, 165, 1991, USA). That means the selectivity is reduced as the permeability is increased.
In addition, upon observation of performance of numerous polymer separation membranes, it has been revealed that most of the polymer separation membranes do not exhibit a performance higher than a certain level.
That is, for each permeability, there exists an upper limit of the selectivity, along which an inversely proportional relation exists between the selectivity and the permeability all the time.
The reason why such an upper limit exists is a natural result as the hard chains of a glass phase polymer substance screen the gas molecules.
The slope of the upper limit has no relation to a chemical structure of the polymer separation membranes.
For the last 30 years, in order to simultaneously increase the selectivity and the permeability, most researchers in the separation membrane industrial field have exerted their energy on synthesizing a novel polymer (for example, U.S. Pat. No. 5,725,633). However, though many researches have been conducted, there has been no report on polymer separation membranes having a performance exceeding the existing upper limit.
The present invention is directed to a fabrication process of a polymeric gas separation membrane which overcomes the limitation of the upper limit on the basis of a novel concept of a polymer composite instead of synthesizing a new polymeric material.
The polymer composite separation membranes have been frequently used in the separation industries. In most cases, it is related to fabrication of a hollow fiber membrane as a blocking body of a solvent (i.e., U.S. Pat. No. 5,310,415, etc.) and partly it is related to addition of an inorganic substance serving as a filler (Journal of Membrane Science, Vol.133,207).
In such a case, however, the adhesion at the interface and the uniform dispersion are problematic in measuring the performance of separation membranes.
Because of their excellent blocking property, liquid crystalline polymers have been subjected to many researches as a separation material (Journal of Membrane Science, Vol. 94, 67), but only a few researches have been conducted on the composite separation membrane material mixed with a liquid crystalline polymer (U.S. Pat. No. 5,599,380). The reason for this is that most liquid crystalline polymer exhibits incompatibility with respect to the thermoplastic resins, thus forms a separate phase. Therefore, it is very difficult to obtain a uniform dispersion and the adhesion at the interface is poor.
Therefore, an object of the present invention is to provide a process for fabricating a blend allowing a uniform dispersion and having an improved interface adhesion by adding a thermotropic liquid crystalline polymer to a thermoplastic resin and at the same time, adding a compatibilizer so that the compatibilizer works at the interface of the liquid crystalline thermoplastic resin to lower the interfacial tension, to provide a process for fabricating membranes with a desired thickness by performing biaxial drawing and extrusion by a film blowing process, and to produce composite separation membranes for gas separation exhibiting a high selectivity in such a manner that an impermeable liquid crystalline polymer uniformly dispersed in the film is formed in a thin and long disk type, working as an obstacle to a gas permeation to change a curvature for a diffusing gas so that the gas passes through the channel at the interface where the gas is separated due to a difference of the interaction with the compatibilizer.
Another object of the present invention is to optimize dispersion of a thermotropic liquid crystalline polymer by using an optimum amount of the compatibilizer.
Still another object of the present invention is to utilize a film blowing process, a biaxial drawing process, for fabricating thin film membranes to let the dispersed phase have a different axial ratio, so that a three-dimensional thin film turns out to have a two-dimensional geometrical form when the gas permeates.
Yet another object of the present invention is to provide a method for fabricating liquid crystalline polymer blended high selective gas separation membranes.
Still yet another object of the present invention is to provide a method for separating a gas by using the liquid crystalline polymer blended high selective gas separation membranes.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described herein, there is provided liquid crystalline polymer blended gas separation membranes obtained by melting and processing 50xcx9c99 wt % of thermoplastic resin, 1xcx9c50 wt % thermotropic liquid crystalline polymer and 0.1xcx9c10 wt % of compatibilizer for the thermotropic liquid crystalline polymer and the matrix polymer.
To achieve the above objects, there is further provided a method for fabricating liquid crystalline polymer blended gas separation membranes, including the steps of: mixing 50xcx9c99 wt % of thermoplastic resin, 1xcx9c50 wt % thermotropic liquid crystalline polymer and 0.1xcx9c10 wt % of compatibilizer of the thermotropic liquid crystalline polymer; and injecting the resulting mixture in an extrusion die of a film blowing apparatus to fabricate a biaxially oriented film.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.